Adsorption of Thallium(I),Lead(II),Copper(II),Bismuth(III) and Chromium(III) by Electrolytic Manganese Dioxide

Abstract

The adsorption of thallium(I), lead(II), copper(II), bismuth(III) and chromium(III) by electrolytic manganese dioxide (EMD) has been followed between pH 2–5 [except for bismuth(III)), where a single pH of 1.5 was employed] at 20°C and an ionic strength of 0.5, employing a wide range of initial metal ion concentrations from 5 × 10⁻⁵ mol/dm³ to 5 × 10⁻² mol/dm³. The adsorption isotherms all fitted a simple Langmuir adsorption equation quite well. The amount of metal ion adsorbed increased with pH for lead and copper ion. However, for both thallium and chromium, a significant drop in adsorption between pH 3 and 4 was observed, associated with the dominant presence of lower-charged hydroxy species at higher pH values. At pH 4, adsorption was in the order Pb^II > Cu^II > TI^I >> Cr^III, whereas at pH ≤ 2 the order was Bi^III ≤ Pb^II > Tl^I > Cr^III > Cu^II. In the low pH range, which is largely free of hydrolysis species, the adsorption of the isoelectronic bismuth, lead and thallium series appears to be dominated by charge, and charge appears a dominant factor with the transition metal pair also. The adsorption of lead (at pH 4 and 5) and bismuth (at pH 1.5) to samples of EMD subjected to prolonged heat treatment at 110, 400 and 650°C were also examined. Samples dried at 110°C mainly lost only surface water, and showed adsorption characteristics similar to untreated samples. Samples heated at 400°C lost all their combined water but still showed significant adsorption, whereas samples heated at 600°C were converted to Mn₂O₃, and showed very small though measurable metal ion adsorption.